I think it's shocking to suggest a teacher with young children isn't as good as one without.
As a young newly qualified teacher I was childless for the first 5 years. I worked longer hours and spent more time planning, yes but that was because I didn't have the experience I now have. I had different demands on my time and my life - seeing friends, going to the pub. I wasn't always some bright eyed bushy tailed workaholic.
Post kids I actually understand children (and their parents and concerns) far better. I am far more experienced than before. Planning a maths lesson on shape for example will take me half the time as I've done it before and know exactly what works and what doesn't. I have honed and perfected my skills.
I work three days a week and for those 3 days I work, I work bloody hard and do a great job. I am EQUALLY as competent as a teacher without young children, more so in some regards.
As a job share I am not overworked and overwhelmed. I 'work' on my two days off, but I do so at my own pace, at home and am able to look after my kids too. If I worked full time I'd be working every evening and weekend and mentally that would be shit. I'd be unhappy, stressed and I wouldn't stick with it. I am a better teacher 3 days a week than I am full time. Arguably I'd say the same would be true for every teacher but I appreciate some have no choice.
I also agree with a PP about a difficult class having two teachers. This can often be helpful as you share the stress AND can bounce ideas off each other. It is so so draining to have 'that class's and be there day in day out without reprise, knowing that nextdoor is a very different picture. I agree that it often makes sense for these classes to be job shares.